Tubular target and production method

ABSTRACT

A tubular target formed of molybdenum or a molybdenum alloy has an oxygen content of less than 50 μg/g, a density of greater than 99% of the theoretical density, and an average grain size of less than 100 μm. The molybdenum or molybdenum alloy tube may be produced by extrusion. In one embodiment, the molybdenum tube has a backing tube of titanium or titanium alloy. In an embodiment, the molybdenum tube has a varying wall thickness with an increase towards its ends.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of copending application Ser. No.11/581,698, filed Oct. 16, 2006; which was a continuation, under 35U.S.C. §120, of international application No. PCT/AT2006/000406, filedOct. 5, 2006; which claimed the priority, under 35 U.S.C. §119, ofAustrian application AT GM 699/2005, filed Oct. 14, 2005; the priorapplications are herewith incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for producing a tubular target, whichcomprises a tube of molybdenum or a molybdenum alloy with an oxygencontent of less than 50 μg/g, a density of greater than 99% of thetheoretical density, and an average grain size transversely to the axialdirection of less than 100 μm as well as a supporting tube of anon-magnetic material.

A target is understood as meaning the material to be sputtered of acathode atomization system. Rotating tubular targets are known anddescribed for example in U.S. Pat. Nos. 4,422,916 and 4,356,073. Duringthe sputtering, the tubular target rotates about a magnetron located inthe tube. Tubular targets are predominantly used for producing coatingsover large surface areas. The rotation of the tubular target achievesthe effect of uniform erosion of the sputtering material. Tubulartargets therefore have a high utilization rate of the target materialand a long target lifetime, which is of significance in particular inthe case of expensive coating materials, as is the case with molybdenum.The utilization rate for planar targets is around 15 to 40% and fortubular targets around 75 to 90%.

The target cooling performed in the space inside the tubular target ismuch more effective than in the case of planar targets as a result ofthe more favorable heat transfer in the tube, which makes a highercoating rate possible. In order to ensure that no cooling water flowsout even with high target utilization, and also to increase themechanical load-bearing capacity and facilitate fixing in the sputteringsystem, the tubular target is usually connected to a supporting tube orbacking tube. The supporting tube must in this case be of a non-magneticmaterial, in order not to interact with the magnetic field whichdetermines the erosion region.

As mentioned, the use of tubular targets is advantageous wheneversubstrates of a large area are to be coated. In the case of molybdenumas the target material, this is the case for example in LCD-TFTproduction and glass coating.

A multitude of production methods are described for the production oftubular targets. Many of those methods take a liquid phase route, suchas for example continuous and centrifugal casting. The latter isdescribed in German published patent application DE 199 53 470 and itscounterpart U.S. Pat. No. 6,793,784 B1. On account of the higher meltingpoint of molybdenum and the resultant problems with finding a suitablemold material, these ways of carrying out production cannot be used formolybdenum and its alloys.

Tubular targets may also be produced by winding a thick strip around acore and welding the contact regions. However, the weld seam has a muchmore coarse microstructure and pores, which leads to non-uniform erosionand, as a consequence, different layer thicknesses. Moreover, in thecase of molybdenum, the welded region is extremely brittle andconsequently at risk of cracking.

A further tubular target is known from U.S. Pat. No. 4,356,073.Production takes place in this case by the sputtering material beingdeposited on a backing tube by plasma spraying. Even using the vacuumplasma spraying technique, however, completely dense tubular targetscannot be produced with an adequately low gas content. Electrochemicaldeposition, as is used for Cr and Sn, is also not suitable formolybdenum and its alloys.

U.S. Pat. No. 5,435,965 and European patent publication EP 0 500 031 A1describe the production of a tubular target by hot-isostatic pressing.There, a backing tube is positioned in a can, so as to produce betweenthe backing tube and the mold an intermediate space into which powder ofthe target material is filled. After closing the can, it is subjected toa hot-isostatic densification operation. The amount of powder used inrelation to the weight of the finished tubular target is in this caseunfavorably high.

U.S. Pat. Nos. 6,878,250 B2 and U.S. Pat. No. 6,946,039 B2 describe theuse of ECAP (equal channel angular pressing) for the production ofsputtering targets. In the case of molybdenum alloys with comparativelyhigh k_(f) values, this leads to a high level of tool wear.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide a method ofproducing a sputtering target which overcomes the above-mentioneddisadvantages of the heretofore-known devices and methods of thisgeneral type and which on the one hand is inexpensive and on the otherhand produces a product which is uniformly eroded in the sputteringprocess, does not tend to give a locally increased sputtering rate anddoes not lead to any contamination of the substrate or the depositedlayer.

With the foregoing and other objects in view there is provided, inaccordance with the invention, a tubular target, comprising:

a molybdenum tube formed of a metal powder of Mo or Mo alloy, themolybdenum tube having:

-   -   an oxygen content less than 50 μg/g;    -   a density greater than 99% of a theoretical density; and    -   an average grain size transversely to an axial direction of the        tube of less than 100 μm; and

a supporting tube of titanium or titanium alloy connected to themolybdenum tube by way of a material bond connection.

There is also provided a method of producing a tubular target, themethod which comprises the following steps:

providing a metal powder of Mo or an Mo alloy with an average particlesize according to Fisher of from 0.5 to 10 μm;

cold-isostatic pressing the metal powder in a flexible mold using a coreat a pressure p, where 100 MPa<p<500 MPa, for producing a green compactin the form of a tube blank;

producing a tube blank by sintering the green compact at a temperatureT, where 1600° C.<T<2500° C., in a reducing atmosphere or a vacuum;

producing a tube by heating the tube blank to a forming temperature T,where DBTT<T<(T_(s) minus 800° C.) and extruding over a mandrel;

joining the tube to a supporting tube of non-magnetic material andmechanically processing, to form a tubular target of a tube ofmolybdenum or a molybdenum alloy with an oxygen content of less than 50μg/g, a density of greater than 99% of the theoretical density and anaverage grain size transversely to the axial direction of less than 100μm, and the supporting tube.

With the above and other objects in view there is also provided, inaccordance with the invention, a method of producing a tubular targetwhich comprises the following method steps: providing a metal powder ofMo or an Mo alloy with an average particle size according to Fisher offrom 0.5 to 10 μm;

cold-isostatic pressing the metal powder in a flexible mold using a coreat a pressure p, where 100 MPa<p<500 MPa, for producing a green compactin the form of a tube blank;

sintering the green compact at a temperature T, where 1600° C.<T<2500°C., in a reducing atmosphere or a vacuum for producing a tube blank;

working the tube blank and joining at least one steel tube end piece forfixing a supporting tube of austenitic steel blank inside the tubeblank;

producing a composite tube by heating to a forming temperature T, where900<T<1350° C. and co-extruding over a mandrel; and

machining the composite tube for forming a tubular target with a tube ofmolybdenum or a molybdenum alloy with an oxygen content of less than 50μg/g, a density of greater than 99% of theoretical density, and anaverage grain size transversely to an axial direction of less than 100μm, and the supporting tube.

In order to achieve an adequate fine grained structure, sinteringactivity, and consequently density, a metal powder with a particle sizeaccording to Fisher of 0.5 to 10 μm is used. For the production of pureMo tubular targets, Mo powder with a metal purity of greater than 99.9%by weight is advantageously used. If a tubular target is produced froman Mo alloy, either powder mixtures or prealloyed powders are used, theparticle size however likewise lying in the range from 0.5 to 10 μm. Thepowder is filled into a flexible mold, in which the core is alreadypositioned. The core determines the inner diameter of the tube blank,with allowance for the compaction during the pressing operation and thesintering shrinkage. Customary tool steels are suitable as the materialfor the core. After filling the flexible mold with the metal powder andliquid-tight closing of the flexible mold, it is positioned in apressure vessel of a cold-isostatic press. The compaction takes place atpressures between 100 and 500 MPa. After that, the green compact istaken out of the flexible mold and the core is removed. Following that,the green compact is sintered at a temperature in the range from 1600°C. to 2500° C. in a reducing atmosphere or a vacuum. Below 1600° C.,adequate densification is not achieved. Above 2500° C., undesired graincoarsening begins. The sintering temperature to be chosen depends on theparticle size of the powder. Green compacts produced from a powder witha particle size of 0.5 μm according to Fisher can be sintered at asintering temperature of as low as 1600° C. to a density of greater than95% of the theoretical density, whereas for green compacts which areproduced from a powder with a particle size of 10 μm according to Fishera sintering temperature of approximately 2500° C. is required. If thedimensional accuracy of the pressing process is not adequate, which isusually the case, the sintered blank is machined. The outer diameter ofthe sintered blank is in this case determined by the inner diameter ofthe container of the extrusion press. To make it possible for theextruded blank to be positioned unproblematically in the container ofthe extrusion press, the outer diameter of the sintered blank issomewhat smaller than the inner diameter of the container. The innerdiameter is in turn determined by the diameter of the mandrel.

In order to reduce the discharge loss of molybdenum during theextrusion, it is advantageous to mechanically fasten a steel end pieceto one end of the molybdenum tube blank. This mechanical fastening maybe performed for example by means of a screwed or bolted connection. Theouter diameter and the inner diameter of the steel end piece of the tubeblank in this case correspond to the outer diameter and the innerdiameter of the molybdenum tube blank.

For the extrusion, the tube blank is heated to a temperature T, whereDBTT<T<(T_(s) −800° C.). DBTT is to be understood here as meaning theductile brittle transition temperature. At lower temperatures, crackingincreasingly occurs. The upper temperature element is given by themelting temperature (T_(s)) of the molybdenum alloy less 800° C. Thisensures that no undesired grain coarsening takes place during theextruding operation. The initial heating may in this case be performedin a conventional gas or electrically heated furnace (for example arotary hearth kiln), allowance having to be made that the gas flowcontrol is chosen such that the lambda value is neutral or negative. Inorder to obtain higher extrusion temperatures, inductive reheating maybe performed. After the initial heating operation, the tube blank isrolled in a glass powder mixture. After that, the tube blank ispositioned in the container of the extrusion press and pressed over amandrel through a die to the respective outer or inner diameter.

It is advantageous if the extruded tube is subjected to a recovering orrecrystallizing annealing process in a reducing atmosphere or a vacuumat a temperature T of 700° C.<T<1600° C. If the temperature goes belowthe lower limit, the stress reduction is too little. At a temperaturegreater than 1600° C., grain coarsening occurs. The extruded tube ismachined on the outer side of the tube, the end faces and advantageouslythe inner side of the tube.

The molybdenum tube produced in this way is connected to a supportingtube of a non-magnetic material. The outer diameter of the supportingtube corresponds approximately to the inner diameter of the molybdenumtube. Furthermore, the supporting tube reaches beyond the respectiveends of the molybdenum tube. Copper alloys, austenitic steels, titaniumor titanium alloys are to be mentioned as particularly suitablematerials for the supporting tube.

Suitable as connecting methods are both those methods which lead to amaterial bond and those which lead to a form-fitting connection. Onecondition, however, is that the contact area between the molybdenum tubeand the supporting tube be at least 30% of the theoretically possiblearea. If the area is smaller, the heat removal is hindered too much.Also the low coefficient of thermal expansion of molybdenum has to beconsidered. Therefore, the joining temperature must be chosen to be aslow as possible. If, for example, the connection between the molybdenumtube and the supporting tube is performed by a forging process, in thatthe supporting tube is positioned in the molybdenum tube and forged overa mandrel, lowest possible forming temperatures of about 500° C. to 800°C. must be chosen. Furthermore, it is advantageous if the material ofthe supporting tube has a low yield strength, in order that stressesoccurring due to plastic flow can be reduced.

In a further method according to the invention, the molybdenum tubeblank is co-extruded with a blank of the supporting tube. The productionof the molybdenum tube is in this case again based on a metal powderwith an average particle size according to Fisher of 0.5-10 μm. The tubeblank is once again produced by cold-isostatic pressing of the metalpowder in a flexible mold using a core and sintering in the range from1600° C. to 2500° C.

Following the sintering, the tube blank is machined. Inside the tubeblank, a supporting tube blank of an austenitic steel is positioned. Atone or both end pieces of the tube blank, a steel tube end piece isjoined on by a mechanical connection (for example a screwed or boltedconnection). The tube end piece has in this case approximately the innerdiameter and outer diameter of the tube blank. The thickness of the tubeend piece preferably lies in the range from 10 to 100 mm. Fastened inturn to the tube end piece is the supporting tube blank. This fasteningpreferably takes place by a welded connection.

The outer diameter of the supporting tube blank may correspondapproximately to the inner diameter of the molybdenum tube blank or elsebe chosen such that a defined gap is produced between the molybdenumtube blank and the supporting tube blank. A steel powder, preferably ofaustenitic steel, is filled into this defined gap. The composite tubebody produced in this way is heated to a forming temperature of from900° C. to 1350° C. Only tubular targets of molybdenum alloys which canbe appropriately deformed at this temperature can be produced in thisway. A higher extrusion temperature cannot be chosen on account of thesteel.

The composite tube blank produced in this way is extruded over a mandrel(co-extrusion), whereby a composite tube is produced. Optionally, thismay be followed by carrying out an annealing process, the annealingtemperature preferably lying around 800° C. to 1300° C.

Use of a gap with a steel powder fill lying in between has the effect ofimproving the bond between the supporting tube and the molybdenum tubeduring the co-extrusion. A gap width of from 3 mm to 20 mm proves to beadvantageous.

The use of glass powder as a lubricant achieves the effect of anoutstanding surface of the tubular target in the case of both extrusionand co-extrusion, whereby the machining can be reduced to a minimum.Furthermore, this ensures that the tubular target is free from pores andalso free from grain boundary cracks. The range from 40 to 80% has beenfound to be an advantageous degree of forming during the extrusionprocess. The degree of forming is in this case determined as follows:((initial cross section before extrusion less cross section afterextrusion)/initial cross section)×100.

After the extrusion/co-extrusion process, it may be advantageous for theextruded tube to be straightened. This can be performed by a forgingprocess over a mandrel.

Furthermore, the wall thickness over the length of the molybdenum tubeor composite tube can also be varied by a subsequent forging process.The wall thickness can in this case be advantageously made thicker inthe region of the tube ends. The region of the tube ends is also theregion of greatest erosion during use.

The surface quality and the dimensional tolerances are set byappropriate machining.

It is ensured by the method according to the invention that the oxygencontent in the molybdenum alloy is <50 μg/g, preferably less than 20μg/g, the density is greater than 99% of the theoretical density,preferably greater than 99.8% of the theoretical density, and theaverage grain size transversely to the axial direction is less than 100μm, preferably less than 50 μm. The average grain size is determinedtransversely to the axial direction because in the case of an deformed,non-recrystallized microstructure, the grains are stretched in the axialdirection and consequently an exact determination of the grain size inthe axial direction is made more difficult. With both methods described,it is possible to produce molybdenum tubular targets with a metallicpurity of greater than 99.99% by weight. Metallic purity is to beunderstood in this case as meaning the purity of the molybdenum tubulartarget without gases (O, H, N) and C. Tungsten is also not considered inthis value, which is uncritical for the application.

For the tubular target according to the invention to be used in the areaof TFT-LCD production, molybdenum alloys which contain 0.5 to 30% byweight of V, Nb, Ta, Cr and/or W are also particularly suitable.

Other features which are considered as characteristic for the inventionare set forth in the appended claims.

Although the invention is illustrated and described herein as embodiedin tubular target, it is nevertheless not intended to be limited to thedetails shown, since various modifications and structural changes may bemade therein without departing from the spirit of the invention andwithin the scope and range of equivalents of the claims.

The construction and method of operation of the invention, however,together with additional objects and advantages thereof will be bestunderstood from the following description of specific embodiments andthe following specific example.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Example

MoO₃ powder was reduced in a two-stage reduction process at 600 and1000° C. to give Mo metal powder with a grain size of 3.9 μm. In arubber tube closed at one end, with a diameter of 420 mm, a steelmandrel with a diameter of 141 mm was positioned in the centre. Themolybdenum metal powder was filled into the intermediate space betweenthe steel core and the rubber wall.

This was followed by closing the rubber tube at its open end by means ofa rubber cap. The closed rubber tube was positioned in a cold-isostaticpress, and compacted at a pressure of 210 MPa.

The green compact had a density of 64% of the theoretical density. Theouter diameter was approximately 300 mm. The green compact produced inthis way was sintered in an indirect sintering furnace at a temperatureof 1900° C. The sintered density was 94.9% of the theoretical density.

After the sintering operation, the tube blank was machined on all sides,the outer diameter being 243 mm, the inner diameter 123 mm and thelength 1060 mm. The extrusion took place on a 2500 t indirect extrusionpress. The tube blank was heated to a temperature of 1100° C. in agas-heated rotary hearth kiln. The lambda value was in this case setsuch that the atmosphere was slightly reducing, whereby oxidation of themolybdenum was prevented. After the initial heating in the rotary kiln,the extruded blank was inductively heated to a temperature of 1250° C.and rolled in a loose fill of glass powder, so that glass powder adheredto the outside on all sides.

This was followed by pressing over a mandrel, whereby an extruded tubewith a length of 2700 mm, an outer diameter of 170 mm and inner diameterof 129 mm was obtained.

A supporting tube, also referred to as a backing tube, of an austeniticsteel with a wall thickness of 6 mm was positioned in the extruded tube.This assembly was straightened over a mandrel on a three-jaw forgingmachine at a temperature of 500° C. and slightly deformed, whereby abond between the molybdenum tube and the supporting tube was produced.

1. A tubular target, comprising: a molybdenum tube formed of a metalpowder of Mo or Mo alloy, said molybdenum tube having: an oxygen contentless than 50 μg/g; a density greater than 99% of a theoretical density;and an average grain size transversely to an axial direction of saidtube of less than 100 μm; and a supporting tube of titanium or titaniumalloy connected to said molybdenum tube by way of a material bondconnection.
 2. The tubular target according to claim 1, wherein acontact surface between said molybdenum tube and said supporting tubeamounts to at least 30% of a theoretically possible surface area.
 3. Thetubular target according to claim 1, wherein said molybdenum tube has anoxygen content of less than 20 μg/g.
 4. The tubular target according toclaim 1, wherein said molybdenum tube has a wall thickness that variesover a length of said tube.
 5. The tubular target according to claim 1,wherein said molybdenum tube has a wall thickness increasing towards theends of said tube.
 6. The tubular target according to claim 1, whereinsaid molybdenum tube has a density greater than 99.8% of the theoreticaldensity.
 7. The tubular target according to claim 1, wherein saidmolybdenum tube has an average grain size transversely to the axialdirection less than 50 μm.
 8. The tubular target according to claim 1,wherein said molybdenum tube consists of pure molybdenum with a metallicpurity, exclusive of tungsten, of greater than 99.99% by weight.
 9. Thetubular target according to claim 1, wherein said molybdenum tubeconsists of a molybdenum alloy containing 0.5 to 30% by weight of atleast one of V, Nb, Ta, Cr, and W.
 10. The tubular target according toclaim 1, wherein said molybdenum tube has the characteristics of a tubeformed by extrusion over a mandrel.
 11. The tubular target according toclaim 1, wherein said molybdenum tube has a recovered or recrystallizedmaterial structure.
 12. The tubular target according to claim 1, whereinsaid molybdenum tube is an extruded tube with a degree of forming of 40to 80%.
 13. A deposition process, which comprises: providing a tubulartarget according to claim 1 and installing the tubular target in asputtering deposition system; and sputtering the tubular target todeposit a molybdenum or molybdenum alloy layer of an LCD-TFT flatscreen.
 14. A deposition process, which comprises: providing a tubulartarget according to claim 1 and installing the tubular target in adeposition system; and sputtering the tubular target to depositmolybdenum or molybdenum alloy on glass to form a coated glass.